Secondary or recurrent caries is considered to be a caries lesion developing adjacent to a restoration. The basic etiology of caries is recognized to lie within the plaque biofilm. This biofilm, due to the frequent ingestion of fermentable carbohydrates, changes from a state of equilibrium with the tooth tissue to a demineralization promoting state. Although the basic etiology of recurrent caries is no different from that of primary caries, there are several factors that make recurrent caries special. The presence of a non-biological material at the site of the process may influence this process in several ways: 1) the interface / gap between the material and the tooth may create a new biofilm site and/or change the diffusion behavior; 2) the material properties, such as a lack in buffering capacity, may influence the development and composition of the biofilm and/or the chemical process. In this project we focus on the biofilm in relation to recurrent caries lesion development both in vivo and in situ. The characteristics we are most interested in regarding the biofilm are its composition and acid production. Biofilm composition will be studied both using traditional culturing methods, and using the most modern DNA-based molecular microbiology technique: pyrosequencing. The in vivo study will allow us to look at differences between primary and secondary lesion biofilm, for the most common restorative materials composite and amalgam. Two in situ studies will allow us to evaluate the effect of restorative material properties on biofilm composition and function and lesion development. Also, the role of gaps between composite restoration and tooth can be evaluated, again with respect to both biofilm composition and function and lesion development. Materials with anti-bacterial and/or anti- caries components will be included in the studies. In this way we hope to elucidate the role of material properties and restoration interface conditions on recurrent caries, hopefully pointing the way towards improving composite restoration performance.